Bonded amalgams and their use in clinical practice

Decreased microleakage

Microleakage is ‘the passage of bacteria, fluids, molecules or ions between a cavity wall and the material applied to it’.16 Its clinical relevance is that the passage of bacteria at the tooth restoration interface may cause recurrent caries or pulpal irritation with subsequent pulpal inflammation.17,18 This could lead to a clinical diagnosis of reversible pulpitis or loss of vitality of the tooth. Operative intervention would then be necessary which, at best, would require a replacement restoration with the cavity inevitably increasing in size19 or, at worst, endodontics or extraction of the tooth.

Microleakage has been traditionally studied in vitro by using a dye leakage model. These studies have shown significantly less microleakage in restorations where either Copal varnish or a resinous material had been applied.20–23 Other studies have compared resin-lined and varnish-lined amalgams and concluded that bonded amalgam leaked less than varnish-lined restorations.11,24 The same conclusion was arrived at in an in vivo study.25

Resin-modified glass ionomer cements have also been advocated for bonding. Two studies in the literature contradict each other, with one26 concluding that these agents should not be used routinely to control microleakage, as increased dye penetration was observed, and the other8 concluding that resin-modified glass ionomers could be used successfully as they significantly decreased microleakage.

A study that examined various resin composite adhesive combinations in reducing microleakage27 concluded that a dentine-bonded system, combined with a high copper amalgam alloy, can provide significant protection against microleakage for up to a year. Interestingly, however, the combination of Tytin amalgam (Kerr, Romulus, MI, USA) and Panavia Ex (Kuraray, Osaka, Japan) showed a significant increase in microleakage scores compared to the other combinations tested.

The effect of amalgam alloy morphology (namely spherical and admixture varieties) was significant as a reduction in microleakage at the cavity margins was seen in all the adhesive groups tested, except where Copal varnish was used to line cavities.28 There was no significant difference in total microleakage scores between the alloy types. Bonding using Amalgambond and Disperalloy (Dentsply/Caulk, Milford DE 19963–0359) and Amalgambond and Tytin showed significantly less microleakage for the admix alloy (Disperalloy) and Amalgambond combination.29

The importance of a coronal seal in endodontics is well recognized.30 Great emphasis has therefore been placed on the quality of the definitive restoration after endodontic therapy.31 In addition, the use of an adhesive system has been promoted to restore endodontically treated teeth (Figure 1) in an attempt to decrease or eliminate microleakage, thus preventing reinfection of the root canal system.32

Decreased incidence of recurrent caries

Recurrent or secondary caries can develop between the tooth and the restorative material with time if fermentable carbohydrate ingestion has occurred and the oral conditions are conducive. Three studies have documented secondary caries as the most common reason for replacement of restorations.33–35 Indeed, marginal microleakage is thought to be a major factor in the development of secondary caries.5,6,36–39 The development of the ability to bond restorations in situ may well improve their marginal integrity. This could in turn retard the progression of caries in this interface. Belcher and Stewart40 reported a lower incidence of recurrent caries around bonded amalgam restorations after two years of clinical service compared to those placed conventionally.

Decreased post-operative sensitivity

The use of adhesive resins to bond the amalgam to tooth tissue offers the potential to decrease post-operative sensitivity41 and, whilst laboratory studies have demonstrated that significant reductions in microleakage may be achieved, post-operative sensitivity can only be demonstrated by in vivo studies. Most published studies have shown no difference between bonded and conventionally placed amalgam restorations with regards to post-operative sensitivity,42–44 but one study did show that bonded amalgams were less sensitive up to six months after placement.45

Increased fracture resistance of the tooth

It has been reported that root-filled posterior teeth are more susceptible to fracture than teeth with intact pulps. This is mainly due to the removal of (much) tooth structure during the endodontic and restorative procedures,46,47 but also due to the dehydration of the dentine that occurs following the completion of the endodontic treatment.48 The use of adhesive techniques, in combination with the definitive amalgam restoration, may therefore afford some support to the weakened tooth and increase the fracture resistance of the tooth.49

In vitro studies, to date, show conflicting results. One study50 reported no significant difference in fracture strength across the experimental groups, and another51 reported that the conventional amalgam group exhibited the weakest resistance to fracture when compared with the groups bonded using amalgam or composite resin, but there was no statistically significant difference between the bonded restorations.

Other in vitro studies found no statistical difference in fracture resistance between bonded and non-bonded amalgam restored teeth,52,53 whilst others have reported variable tooth strengthening of bonded amalgam restorations with different adhesive products,54,55 and it has also been reported that fracture resistance is markedly increased in bonded amalgam restorations.56,57

Unsupported enamel has traditionally been removed when amalgam is planned to restore the tooth, as the material confers no strength to the remaining tooth3 and this may lead to fracture of the (weak) tooth. If the restoration was bonded in situ, then it may obviate the need for tooth tissue removal (Figure 2). Latino, Troendle and Summitt58 assessed whether a bonded amalgam restoration did actually support unsupported enamel. In this study, no significant difference was detected between the performance of the bonded and conventionally placed (control) amalgam groups. The authors therefore recommended that restorative materials should not be relied upon to support undermined occlusal enamel. In contrast, Franchi et al59 concluded that bonded amalgam appears to be as effective as composite in supporting undermined enamel in terms of resistance to fracture.

The amount of tooth tissue remaining following cavity preparation may have an influence upon the fracture resistance of the tooth. The greater the amount of tooth tissue missing then the greater the risk of fracture. Lindemuth, Hagge and Broome60 studied bulk fracture strengths of teeth with large and small cavities. The large amalgam restoration group showed no difference in the incidence of bulk fracture between the conventionally placed and those restored with bonded amalgam restorations. However, the small restoration group displayed a significantly greater bulk fracture strength when the restoration was bonded. It was concluded that other factors could also influence fracture resistance.

Decreased cuspal deflection

The ability of the restoration to bond to its surrounding tooth tissue may brace the weakened tooth and decrease its flexion, especially when a large amount of tooth is missing. El-Badrawy found, in an in vitro study,61 that bonding amalgam restorations decreased the cuspal deflection of maxillary premolars and consequently may assist in restoring strength to the tooth.

The challenge of how to deal effectively with cracked cusps is a problem in clinical practice. By utilizing adhesive technology, the flexure around a fracture line may be decreased or even eliminated62 and the successful treatment of cracked cusp by such an approach has been reported.63 This is illustrated in Figure 3. A fracture line extends from the distal marginal ridge onto the floor of the cavity and it was treated by bonding the amalgam in situ in an attempt to decrease the flexure of the tooth and increase the fracture resistance of the tooth.

Conservation of tooth tissue

In the past, cavity design with mechanical retention was required for traditional amalgam restorations at the expense of healthy tooth tissue.11 Current techniques aim to remove the minimal amount of diseased tooth tissue necessary and this is facilitated by the use of adhesive technology. Staninec64 tested bonded amalgams without undercuts against conventionally placed amalgams in undercut cavities, and showed that the bonded group showed a higher resistance to dislodgement and found the bonding technique helped conserve tooth tissue. Setcos et al65 noted that a few ‘minimally retentive’ conventional restorations were lost early which, when replaced using an adhesive technique, appeared to hold well without the additional mechanical retention. Figure 4 shows a cavity prepared to receive a bonded amalgam restoration. The red line depicts the amount of tooth tissue lost had a traditional cavity been cut.66

The use of an adhesive may prevent the needless removal of sound tooth tissue to create traditional resistance and retention form (Figures 2, 4 and 5), or the use of dentine pins with their attendant placement hazards.67 Summitt et al68 showed that there was no significant difference in failure rate and no difference in the performance of pin-retained amalgam restorations and bonded amalgam restorations at six years.